1. Field of the Invention
This invention relates to a process and apparatus for locating electrically conductive terminal pads or contact points on a substrate and more particularly to a process and apparatus for using two probes to test line continuity, capacitance and related electrical measurements on a distorted or non-uniform ceramic substrate.
2. Description of Related Art
Electrical testers are used in the electronics packaging industry to detect the presence of defects which affect the electrical performance of the semiconductor package. The defects tested for can include circuit line opens and shorts, near opens and near shorts.
In testing the functionality of an integrated circuit in a semiconductor or other substrate, probes are used to engage electrically conductive terminal pads or contact points. Precise alignment of a probe relative to a terminal pad or contact point is required in order to ensure electrical contact between the probe and the contact point. In many cases, after processing and heat treatment, the contact points on a substrate are not located at the precise coordinate specified in the design, but have shifted to a new location due to design factors or processing conditions. This phenomenon is very common in ceramic packaging. If the positional error is large enough, the probe will not make electrical contact with a designated contact point and the tester will report an open in the network under test. Since invalid opens can be a common occurrence, it is assumed that the initial open is due to a probe not making contact with the contact point and a probe adjustment-retest loop is initiated. The probe adjustment-retest loop is either a manual repositioning of the probe or software controlled search routine. A manual search relies upon intervention from the test operator to reposition the probes in the direction of the contact point, while a software controlled routine automatically steps the probes a preset distance and direction in an attempt to locate the contact point. The probe adjustment-retest loop continues until either the operator is convinced that the probes are contacting the contact points and the open fault indication is valid, or the search routine is completed.
The manual process for locating contact points is inefficient because it relies upon intervention from the operator to determine the actual contact point location and manually reposition the probes. This can be time consuming, labor intensive and prone to human error. The software controlled routine, while effective in performing a single point capacitance test, is less effective when attempting to perform a two point resistance measurement because the software controlled routine shifts each probe by a preset distance in the same direction. Thus, if one probe is in contact with its respective contact point and the other probe is not contacting the designated contact point, the probe making contact may be moved off it's contact point during the retest. If both probes are constrained to move the same direction and distance, they may never contact their respective contact points simultaneously and a good part may be discarded because of an invalid fault indication.
A further problem frequently encountered in substrate electrical testing is how to efficiently test products which have terminal pad sizes of less than 100 um. As the size and pitch of semiconductor input-outputs continue to shrink with the implementation of advanced ground rule designs, the task of electrically probing the terminal pads on a substrate becomes increasingly more difficult. Additionally, as the density of the terminal pads on a substrate increases so that the terminal pads are closer to each other and smaller in size, precise alignment of the probes with the terminal pads becomes even more difficult.
Probe tools and substrate inspection apparatus have been designed to attempt to overcome the difficulties associated with changes in substrate size and terminal pad position. For example, U.S. Pat. No. 5,491,427 discloses a probe and electrical part/circuit inspecting apparatus and method. The probe is a grid pattern of contact bump electrodes and flexibly accommodates certain changes in the position and size of the device under test. The probe allows for the testing of a wide range of semiconductor devices having various sizes and terminal arrangements assuming a regular pattern and pitch of the device terminals. The apparatus does not allow independent movement of the probe electrodes and does not provide for non-uniform distortion of the position of the terminal pads.
U.S. Pat. No. 3,963,985 discloses a probe device having multiple probe heads and a method to adjust the distance between probe heads to accommodate changes in the substrate size after processing. The probe heads are mounted on a structure having a known thermal coefficient of expansion. The structure is expanded or contracted to reposition the probe heads corresponding to changes in the dimensions of the substrate after processing. It is necessary to measure the substrate dimensions after processing and the device accommodates only uniform error and does not search for terminal pads.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method and apparatus which enables a probe tester to determine if a probe is making contact with a designated contact point and search for and locate the contact point if the probe is not making contact.
It is another object of the present invention to provide a method and apparatus which enables independent movement of probes relative to contact points on a substrate during a software search routine.
It is another object of the present invention to provide a method and apparatus for precisely aligning a probe with a contact point on a substrate.
It is yet another object of the present invention to provide a method and apparatus to efficiently test continuity of circuit lines and continuity between contact points on a substrate.
A further object of the invention is to provide a method and apparatus for efficiently testing semiconductor products which have significant non uniform terminal pad positional error.
A further object of the invention is to provide a method and apparatus to facilitate the testing of semiconductor substrates having small terminal pad size and positional error by eliminating extra processing steps such as mapping the location of terminal pads after processing.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.